“While I deeply respect General Mattis’ service, I will oppose a waiver,” Gillibrand said in a statement, referring to the law that prevents those on active military duty within the last seven years from serving in the civilian post.

Congress will have to confirm Mattis, and also pass a law providing the waiver, a provision that dates back to 1947.

“Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule,” Gillibrand said.

Sen. John McCain (R-Ariz.), chairman of the Senate Armed Services Committee, has endorsed Mattis’ nomination, implying an openness to supporting the waiver.

The swift objection from New York’s junior senator comes just weeks after she began signaling possible interest in running for president in 2020.